1. Field of the Invention
The present invention relates to a method for solvent extraction of phosphoric acid from a highly concentrated impure phosphoric acid solution containing at least 45 wt.% of P.sub.2 O.sub.5, in high yield by a counter current extraction.
Certain terms used in the specification will be defined as follows:
The "water solubility wt.%" represents "an amount in gram of water/100 g. of the solution at room temperature".
The "free sulfuric acid concentration by mol/l" is a half of the value obtained by subtracting the value of the H.sub.3 PO.sub.4 concentration (mol/l) from the value of the H.sup.+ concentration (gram ion/l) obtained by titration with an aqueous sodium hydroxide solution using Bromophenol Blue as an indicator.
The "wet process phosphoric acid" means a phosphoric acid obtained by decomposing a phosphate rock with sulfuric acid and removing solids by filtration, or by adjusting its concentration or by subjecting it to pretreatment such as removal of sulfates, silicates, fluorine or organic matters.
The "extraction battery" means a battery for extracting phosphoric acid into a solvent phase by counter-currently contacting a solvent with an aqueous impure phosphoric acid solution and an aqueous phase discharged from the purification battery. The present invention is concerned with this extraction battery.
The "purification battery" means a battery for extracting impurities contained in the extracted solution into an aqueous phase by counter-currently contacting the extracted solution from the extraction battery with high purity water or aqueous phosphoric acid solution.
The "wash solution" means the above-mentioned high purity water or aqueous phosphoric acid used in the purification battery.
The "stripping battery" means a battery for obtaining an aqueous solution of purified phosphoric acid or phosphate by counter-currently contacting the solvent phase from the purification battery with high purity water or aqueous alkali solution.
The solvent for extracting phosphoric acid from a highly concentrated impure phosphoric acid solution by a counter-current method (the solvent being hereinafter referred to as "a solvent for a highly concentrated solution"), must form two phases when contacted with such a phosphoric acid solution. As such a solvent, there are known, for instance, aliphatic alchols having from 6 to 10 carbon atoms, ketones such as methyl isobutyl ketone, ethers such as diisopropyl ether and alkyl phosphates such as tributyl phosphate. However, with these solvents (except for the alkyl phosphates), the yield of phosphoric acid was at best as low as from 50 to 60%. In order to increase the yield by counter current extraction, it is necessary that adequate extraction of phosphoric acid must even at the location near the outlet of the raffinate where the concentration of phosphoric acid is naturally low. However, known types of solvents for a highly concentrated solution normally have an extremely poor ability for extracting phosphoric acid in this low concentration region. For instance, 2-ethyl hexanol having 8 carbon atoms forms two phases when contacted with a wet process phosphoric acid containing even 50 wt.% of P.sub.2 O.sub.5, but it is scarcely capable of extracting phosphoric acid from an aqueous phase containing less than 20 wt.% of P.sub.2 O.sub.5.
On the other hand, n-butanol, isobutanol, etc. are known as solvents which are capable of adequately extracting phosphoric acid at the low concentration region. However, these solvents do not form two phases when contacted with a wet process phosphoric acid containing at least 45 wt.% of P.sub.2 O.sub.5 as they tend to be homogeneously mixed therewith, and accordingly, they are not suitable as solvents for a highly concentrated solution.
Further, in general, the higher the phosphoric acid concentration, the lower the selectivity for phosphoric acid in the extraction. Solvents for a highly concentrated solution have relatively good selectivity. Nevertheless, their selectivity is still dependent on the concentration of the phosphoric acid. For instance, even when a highly concentrated impure phosphoric acid solution was contacted with methyl isobutyl ketone and then the extracted solution was subjected to treatments by the purification and stripping batteries, a highly pure aqueous phosphoric acid solution was not obtainable and it was necessary to further purify it by e.g. crystallization (Japanese Unexamined Patent Publication No. 99993/1974 or U.S. Pat. No. 3,914,382).
Even in such a case, if a greater amount of the wash solution is used, it is possible to increase the purity of the phosphoric acid solution obtained from the stripping battery. However, the phosphoric acid in the aqueous phase discharged from the purification battery is recovered by supplying the aqueous phase to the extraction battery, and accordingly the amount of the aqueous phase naturally increases leading to a decrease of the concentration of the aqueous phase in the extraction battery, and accordingly leading to a decrease of the extraction yield of phosphoric acid. In order to prevent the decrease of the yield, it is necessary to increase the amount of the solvent which has a low extraction ability, depending upon the increase of the amount of the aqueous phase, and accordingly the amount of the extracted solution increases and the amount of the wash solution to be contacted therewith will have to be further increased.
The difficulty resulting from the decrease of the selectivity for phosphoric acid may be overcome if the ability of the solvent to extract phosphoric acid can be increased by certain means. In this case, even when the amount of the aqueous phase entering from the purification battery to the extraction battery increases, it is necessary only to sacrifice the high yield to some extent or to increase the amount of the solvent to some extent.
The following methods are known as methods for increasing the yield of phosphoric acid from a highly concentrated impure phosphoric acid solution.
(i) A method in which sulfuric acid is added to the feed impure phosphoric acid solution.
According to this method, the yield can be increased only by 7 to 8%. Further, there is a problem that the added sulfuric acid is extracted together with phosphoric acid.
(ii) A method in which sulfuric acid is added to the raffinate and contacted with the solvent, thereby to recover phosphoric acid contained in the raffinate (Japanese Patent Examined Publication No. 4279/1977 or U.K. Patent No. 1199042).
According to this method, a high yield is obtainable. However, the phosphoric acid concentration in the extracted solution is low. Besides the extracted solution contains sulfuric acid, and it is difficult to recover phosphoric acid therefrom.
(iii) A method in which a mixture of a dialkyl ether having from 4 to 10 carbon atoms and an aliphatic alcohol having from 3 to 8 carbon atoms is used as the solvent (Japanese Unexamined Patent Publication No. 70294/1975 or U.S. Pat. No. 3,903,247).
According to this method, the yield is at best 80%.
We have made extensive studies with a technical objective to develop a method whereby it is possible to extract phosphoric acid from a highly concentrated impure phosphoric acid solution containing at least 45 wt.% of P.sub.2 O.sub.5 in higher yield than the conventional methods, e.g. as high as at least 90%, and at the same time to substantially prevent the coextraction of sulfuric acid which is added to increase the yield.
Firstly, we studied the behaviours of phosphoric acid and sulfuric acid in the extraction. The data thereby obtained are shown in the Table below.
______________________________________ Free sulfuric acid concentra- Solubility of tion in the water in the Distribution aqueous phase solvent.sup.1 coefficient of (mol/l) (wt. %) phosphoric acid.sup.2 ______________________________________ 0.5 2.0 0.001 3.0 0.020 5.0 0.043 8.0 0.062 1.0 2.0 0.002 3.0 0.048 5.0 0.076 8.0 0.105 2.0 2.0 0.005 3.0 0.090 5.0 0.125 8.0 0.210 3.0 2.0 0.012 3.0 0.150 5.0 0.242 8.0 0.365 ______________________________________ Note: .sup.1 As the solvent, a solvent mixture of methyl isobutyl ketone and nbutanol was used. The solubility of water was adjusted by changing the mixture ratio. .sup.2 H.sub.3 PO.sub.4 concentration in the solvent phase/H.sub.3 PO.sub.4 concentration in the aqueous phase, where the H.sub.3 PO.sub.4 concentration in the aqueous phase was 2 mol/l.
From the results, the following facts have been confirmed.
(1) The distribution of phosphoric acid into the solvent is higher as the sulfuric acid concentration is higher.
(2) The distribution of phosphoric acid into the solvent abruptly decreases when the solubility of water in the solvent becomes lower than a certain level. On the other hand, the higher the solubility of water, the higher the distribution. However, if the solubility of water is too great, the phase separation becomes difficult or impossible. Among the systems wherein the solubility of water is the same, the phase separation is better in a system where a mixture of solvents is used than a system where a single solvent is used.
(3) The distribution of sulfuric acid into the solvent is greater as the phosphoric concentration is higher.
(4) The distribution of sulfuric acid into the solvent is not very much affected by the solubility of water so long as the solubility of water is maintained within a certain range. However, among the systems wherein the solubility of water into the solvent is the same, the distribution of sulfuric acid into the solvent is smaller in a system wherein a mixture of solvents is used as the solvent, than in a system wherein a single solvent is used.
From the foregoing facts, the following conclusions have been reached.
(5) In order to prevent the coextraction of sulfuric acid, it is necessary to lower the sulfuric acid concentration in the battery solution in the place near the outlet of the extracted solution having a high phosphoric acid concentration. Accordingly, in order to increase the yield of phosphoric acid, it is necessary to increase the sulfuric acid concentration at the place near the outlet of the raffinate having a low phosphoric acid concentration.
(6) The solvent must be selected so that the solubility of water therein falls within a certain range.
(7) A mixture of solvents is better than a single solvent from the viewpoints of the phase separability and the selectivity for phosphoric acid.
On the basis of the foregoing knowledge, we have conducted a further study and have found that it is possible to accomplish the above-mentioned technical objective, and thus, the present invention has been accomplished.